I got confirmation from the resident IZHMASH expert that you do indeed need to mill the front of the magazine well to fit, and you also need to mill away the sides for better clearance. When fitting it by hand it is obvious where to machine it away.

Its rainy here today so I may piddle with it some - still a bit too cold to make pistol grips

As pricey as the Russian latch. They do have that latch in stock too. Chokes, latches, everything but the "paradox" rifled choke I think I'm going to make a choke adapter just for a rifled and forget about ever getting that factory choke.

One thing to keep in mind is that the lever will extend into the magazine well. This can block you from engaging the latch when a magazine is loaded. If you look at factory mags, and the factory magwell, they have a bevel that lets the latch engage. I'm pretty sure you'll need to grind on the aftermarket magazines to get them to fit like factory.

You know, now that I look at it, there's a recess for the folding stock hinge pin on the bottom of the magwell.

OK A few things... buddy came over with his mags and we did some comparison. The front locking lugs on the different magazines are NOT all the same:

We were surprised that the promag fits with a side-folding latch as-is.

We also noticed that the blueprint location for the latch pin is a little too far forward. Not enough to not work, but if you compare the recess in the factory magwell to the location you are supposed to drill on the receiver, it appears to be just a bit too far forward. It could go back about .010" and still be OK, and it will probably engage better that way.

More testing to come - going to the range today to do some more pistol grip reliability testing